1. Field of the Invention
The present invention relates to an optical transmission line and an optical transmission system for transmitting an optical signal in a wavelength band of 1.0 μm.
2. Description of the Related Art
A holey fiber or a photonic crystal fiber is a new type of an optical fiber that includes a core region at its center and a cladding region arranged on an outer circumference of the core region. The cladding region includes a plurality of air holes around the core region to propagate light in the core region by lowering average refractive index of the cladding region using the air holes and by utilizing the principle of total reflection of light. The holey fiber controls the refractive index using the air holes, so that specific characteristics, such as an Endlessly Single Mode (ESM), which has been unachievable by conventional optical fibers, zero-dispersion wavelength, which is shifted toward a side of extremely short wavelengths, or the like, can be realized. The ESM means characteristics in which the cutoff wavelength does not exist and lights at all wavelengths are transmitted in the single mode, so that high-transmission-speed optical transmission can be achieved across a broad bandwidth.
On the other hand, recently a technology for an Ytterbium-doped optical fiber (YDF) usable as an amplifying optical fiber in a wavelength band of 1.0 μm having a wavelength bandwidth from 1000 nm to 1100 nm or 1300 nm has been being matured. Accordingly, there are increasing demands for a fiber laser for the 1.0-μm wavelength band, an optical fiber for an SC light source, and an optical fiber applicable to an optical transmission line and the like. A holey fiber is expected to meet such a demand. For example, in K. Ieda, et al., “Visible to Infrared WDM transmission over PCF”, ECOC2006-Tu3.3.4 (2006), results are reported of experiment on light transmissions across a broad bandwidth including a wavelength of 1064 nm using a holey fiber as an optical transmission line.
In a conventional holey fiber, however, a wavelength dispersion in the 1.0-μm wavelength band is, for example, equal to or less than −20 ps/nm/km, so that an absolute value of the wavelength dispersion is large. Accordingly, when using this holey fiber as an optical transmission line to transmit an optical signal in the 1.0-μm wavelength band, there is a problem that the optical signal severely distorts and its quality degrades. Furthermore, because the conventional holey fiber has a dispersion slope, an optical signal differently distorts depending on the wavelength. Therefore, when using, for example, a wavelength-division multiplexing (WDM) signal as an optical signal, the quality of the optical signal may vary depending on the wavelength of the optical signal. Accordingly, the conventional holey fiber is not appropriate for an optical transmission across a broad bandwidth.